17 Library introduction [library]

17.6 Library-wide requirements [requirements]

17.6.5 Conforming implementations [conforming] constexpr functions and constructors [constexpr.functions]

This standard explicitly requires that certain standard library functions are constexpr ([dcl.constexpr]). An implementation shall not declare any standard library function signature as constexpr except for those where it is explicitly required. Within any header that provides any non-defining declarations of constexpr functions or constructors an implementation shall provide corresponding definitions.